add IMD file reader

doc/using.md

@@ -141,7 +141,26 @@ exact format varies according to the extension:
 	are ignored.
 
   - `.imd`: a disk image format created by [David Dunfield](http://dunfield.classiccmp.org/img/index.htm).
-	These images can be used as input image for writing a disk. If you don’t 	know the diskformat, Fluxengine will show the stored comment and 	display the geometry to use. For input files you normally have to 	specify it yourself.
+	These images can be used as input image for writing a disk. If you don’t 	know the diskformat, Fluxengine will show the stored comment and display 	the geometry to use. Just give a command like:
+
+    ```
+    $ ./fluxengine write ibm -i filenaam.imd
+	Comment in IMD image:
+	*** DOUBLE SIDED FORMAT ***                                   
+	HP-IB,RS-232C & RS-449 PREPROCESSOR                           
+	For HP1650A, HP1651A & HP16500A                               
+	10342-13017                                                   
+	Rev 2749 Ver 1.1                                              
+	Copyright (c) 1987, Hewlett-Packard                           
+                                                              
+                                                               
+	reading IMD image
+	77 tracks, 2 heads; MFM; 250 kbps; 5 sectoren; sectorsize 1024; sectormap 	12345; 770 kB total 
+	Writing to: :d=0:s=0-1:t=0-79
+  	0.0: Error: track data overrun
+    ```
+ 	
+	For input files you normally have to specify it yourself.
 
 
 ### High density disks